A piezo-electric oscillator is used in a mobile communication device such as a portable telephone or a pager because of its small size, high stability without adjusting, and economy of power consumption with using a rechargible battery as a power supply.
One type of a conventional piezo-electric oscillator includes an oscillatory circuit using a piezo-electric resonator and a buffer amplifying circuit for amplifying an output of the oscillatory circuit or buffering changes of a load connected serially with the oscillatory circuit. The oscillatory circuit includes a piezo-electric resonator such as a crystal resonator, an amplifying unit including N-type and P-type MOSFETs which together function as a voltage amplifier, a feedback unit including phase shifting capacitors for feedbacking some proportion of the amplified signal.
In operation, the piezo-electric resonator oscillates at a predetermined frequency to generate an oscillatory signal. The amplifying unit amplifies the oscillatory signal. The feedback unit feedbacks some proportion of the oscillatory signal thus amplified. The oscillatory signal is supplied through the capacitor to the buffer amplifying circuit. The buffer amplifying circuit amplifies the oscillatory signal to be supplied as an output signal to an external circuit.
According to the conventional piezo-electric oscillator, however, there is a disadvantage in that start-up of the oscillation thereof is difficult to be carried out in some cases. The reason thereof will be explained.
Now, amplification degree of the amplifying unit and feedback factor of the feedback unit are defined as .mu. and .beta., respectively. A product .mu..beta. should be larger than 1 to start-up the piezo-electric resonator to oscillate. However, the value of .mu..beta. depends on various factors.
First, the product .mu..beta. changes in accordance with a transfer conductance g.sub.m of the MOSFETs of the amplifying unit if an equivalent resistance CI of the piezo-electric resonator is constant. The product .mu..beta. becomes large as g.sub.m becomes large. However, the power consumption of the piezo-electric oscillator may increase if g.sub.m is set to be large.
Second, the product .mu..beta. depends on the equivalent resistance CI of the piezo-electric resonator. The feedback factor .beta. becomes small as the equivalent resistance CI becomes large. Therefore, the product .mu..beta. becomes small as the excitation level I.sub.Q becomes small. It is preferable for the equivalent resistance CI to change little in accordance with changes of an excitation level I.sub.Q which is an amount of current flowing through the piezo-electric resonator, because the change of the equivalent resistance CI may cause instability of characteristics of the piezo-electric oscillator such as a temperature characteristic. In practice, however, the equivalent resistance CI depends on physical design thereof and becomes large as the excitation level I.sub.Q becomes small. In other words, the equivalent resistance CI is large at the start-up of the oscillation of the piezo-electric resonator when I.sub.Q is nearly zero.
If capacities of the phase shifting capacitors and the transfer conductance g.sub.m of the MOSFETs are determined so that the product .mu..beta. has an adequate value (larger than 1) when the equivalent resistance CI is in stationary state, the product .mu..beta. is smaller than 1 at the start of oscillation. In the stationary state, the growth of amplitude of the oscillation saturates (.mu..beta.=1), and the excitation level I.sub.Q has a value where the gain of the amplifying unit 14 and the loss of the feedback unit are balanced. Though the product .mu..beta. becomes larger than 1 if the excitation level I.sub.Q is significantly large, there are some disadvantages such as deterioration of aging characteristic of the piezo-electric resonator or increase of the power consumption thereof.
The feedback factor .beta. becomes large as capacitance of the phase shifting capacitors becomes small, so that the product .mu..beta. becomes large as capacitance of the phase shifting capacitors becomes small. However, sensitivity of operation point of the piezo-electric resonator becomes high as the capacitance of the phase shifting capacitors become small, so that frequency stability of the piezo-electric resonator deteriorates.
Thus, the conventional piezo-electric oscillator may be difficult to oscillate because of a small value of .mu..beta. if a piezo-electric resonator having a characteristic in which an equivalent resistance CI is high at start-up of oscillation thereof.